1. Field of the Invention
The present invention relates to a variable speed drive unit for a continuously variable transmission, in particular a CVT transmission. The function of the variable speed drive in such a transmission is to transmit the torque of an input shaft to an output shaft, whereby the transmission ratio condition between the input shaft and the output shaft is freely adjustable within a transmission ratio range.
2. Description of the Related Art
In known variable speed drives for continuously variable transmissions, chains or metal bands are used, which are engaged for example with conical transmission surfaces on the input and output shafts.
It is also known to provide variable speed drives for CVT transmissions without chains or metal bands. For example, there are so-called “half-toroidal CVTs,” in which the motion is transmitted between two toroidal surfaces by means of an adjustable intermediate disk. In that case the transmission of power necessitates high contact pressure forces, which leads, in turn, to large, increasing reaction forces in the element pairings of the variable speed drive. Therefore, in such variable speed drives that transmit torque between two toroidal surfaces a plurality of intermediate disks are mounted between the toroidal disks, and/or a plurality of variable speed drives are mounted in parallel, so that the high contact pressure forces can be reduced and/or higher torques can be transmitted.
An alternative is a so-called conical ring variable speed drive, which also manages without a chain or metal band, but works instead with a rigid transfer ring. The torque is thereby transmitted by means of a ring wheel between two opposing cone-shaped surfaces, which are rotatably mounted by means of two parallel axes. The spread of the transmission ratio is limited by the proportion between the smallest and largest diameter of the cone.
An object of the invention is to provide a variable speed drive for a continuously variable transmission, in particular a CVT transmission, which permits a wide spread of the transmission ratio and operates quietly. At the same time, it should be possible to produce the variable speed drive inexpensively.